Lovely portraits, carrion crows are my favourite birds! What you call leucism is in fact some temporary discoloration that has been attributed to some mineral/vitamin deficiency, it is very common in the population in East Park in Hull:
“Partly coloured feathers are very unusual in leucism. Individual feathers that are partly coloured usually indicate a bad condition of the bird during feather growth and is not an inheritable character (ie, is not leucism). This is often seen in, eg, Carrion Crows, especially those eating junk food in cities
You can find more info here (where I got the excerpt):http://www.housesparrow.eu/pdfs/vanGrouwHein2006_NotEveryWhiteBirdIsAnAlbinoSenseAndNonsenseAboutColourAberrationsInBirds.pdf